Biological process

down regulation of regulatory T cell differentiation, down-regulation of regulatory T cell differentiation, downregulation of regulatory T cell differentiation, negative regulation of regulatory T lymphocyte differentiation, negative regulation of regulatory T-cell differentiation, negative regulation of regulatory T-lymphocyte differentiation, negative regulation of suppressor T cell differentiation, negative regulation of suppressor T-cell differentiation

Any process that stops, prevents, or reduces the rate of differentiation of regulatory T cells.

negative regulation of regulatory T cell development

Note that immunologists typically use the word 'development' to refer to cells of B or T cell lineages undergoing the process that GO describes as 'cell differentiation'.